In at least one known CT system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time required for multiple slices, a "helical" scan may be performed. To perform a "helical" scan, the patient is moved while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a one fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed. An image reconstruction algorithm which may be utilized in reconstructing an image from data obtained in a helical scan is described in U.S. patent application Ser. No. 08/436,176, filed May 9, 1995, and assigned to the present assignee.
Certain reconstruction process steps are known to produce noise structures in an image. For example, underscan weighting ("USW"), also known as peristaltic correction of CT projection data, is employed to reduce motion artifacts that results when patient anatomy moves during a 360 degree CT scan. Patient motion causes a discontinuity between the beginning and ending projections which typically produces low frequency streaks in the direction of the scan start angle, i.e., the initial relative angular position of the x-ray source and the subject.
In USW, since a 360 degree scan generates sufficient projection data to reconstruct two independent images of each scanned slice, two such independent images are generated. Specifically, over a small angle, e.g., 45 degrees, the data prior to backprojection is decreasingly weighted with a continuous cubic function so the image contribution of the projection data at the discontinuity is zero. Redundant data, i.e., opposing samples, are increasingly weighted so the contribution of the projection data opposite the discontinuity is assigned a weight of 2. USW thus softens the discontinuity and preserves the reconstruction requirement that the sum of the backprojection weights from every angle be equal.
However, USW has the undesirable effects of producing a noise pattern oriented in the direction of the scan start angle and exposing a patient to unnecessary radiation. The noise occurs because only one projection (N photons) is effectively backprojected in the USW direction, while two projections (2N photons) are used in the orthogonal direction. The projection noise in the USW direction will therefore be 1.414 times greater than in the orthogonal direction. This noise pattern is especially noticeable in large uniform regions such as the liver, and such noise complicates the diagnosis of low contrast lesions in this organ that are of vital interest in oncology patients.
Reconstruction algorithms for helical scanning also require the use of helical weighting ("HW") as a function of view angle. HW is similar to USW and the effect of HW on helical images noise is the substantially the same as USW. That is, with HW, projection noise will be 1.414 times greater in the maximum HW direction.
USW and HW also expose the patient to the same X-ray dose for every projection even though some of the projections contribute almost zero weight to the reconstruction. Even though some projections make substantially no contribution, the patient is exposed to an x-ray dose to collect that subsequently zero weighted data.
X-ray dose is typically controlled by the x-ray tube current ("mA") which flows in the x-ray tube. Traditionally, this current was fixed at a level which provided a constant dose during the entire scan. However, more recently, and to reduce patient dose, the x-ray tube current has been varied during the scan as a function of the projection angle, i.e., the relative angular position of the x-ray source and the subject being x-rayed. One such method is described, for example, in U.S. Pat. No. 5,379,333, entitled "Variable Dose Application By Modulation of X-Ray Tube Current During Scanning", which is assigned to the present assignee and incorporated herein, in its entirety, by reference.
Although varying, or modulating, x-ray tube current as a function of scan angle facilitates reducing patient dose, such variations do not take into account artifacts which may be later introduced due to weighting functions such as the weighting function employed in USW and HW. Of course, in addition to removing motion artifacts, it would be desirable to remove other artifacts from the image.